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Cassini continues its exploration of the Saturn system with the 42-day Rev 198, which begins on September 27 at its farthest distance from the planet. This is also called the orbit's apoapse. At this point, Cassini is 2.62 million kilometers (1.62 million miles) from Saturn's cloud tops. Rev 198 occurs during the first inclined phase, which lasts until March 2015, of the Cassini Solstice Mission. The inclined phase will allow for polar views of Saturn and Titan as well as better vistas of Saturn's rings than those Cassini had while in the earlier, equatorial phase of the Solstice Mission. Forty-one ISS observations are planned for Rev 198 with the majority focused on Saturn's atmosphere and rings and on Titan during the T95 flyby.
On October 5, one week after apoapse, ISS will start off Rev 198 by acquiring a cloud and haze monitoring observation of Titan. Observations like this one are designed to track changes in the distribution of clouds across Titan as well as to monitor changes to its haze layers. The October 5 observation will be taken from a distance of 1.72 million kilometers (1.07 million miles). A similar observation will be taken on October 7 from a distance of 1.30 million kilometers (0.81 million miles). Both observations will cover Titan's north polar region, though the October 7 observation will have a better phase angle for observing the surface. On October 8, ISS will acquire an astrometric observation of Saturn's small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn's larger icy moons. Three more astrometric observations will be acquired on October 10, 11, and 19.
Immediately afterwards, on October 8, ISS will acquire a quick observation of Saturn using the WAC. This observation is part of a series of "Storm Watch" observation sequences designed to take advantage of short, two-minute segments when the spacecraft turns the optical remote sensing (ORS) instruments back to Saturn as a waypoint between other experiments' observations. These sequences include blue, clear, two methane band, and one full-frame, continuum band filter images. Two more sequences will be taken later on October 8. Seven more storm watch observations will be taken during this orbit with five on October 10 and two on October 11. Also on October 8, ISS will ride along with the Visual and Infrared Mapping Spectrometer (VIMS) as it observes an occultation of the red giant star L2 Puppis by the ring system. ISS images will be centered around the time the star passes behind the F ring. On October 10 and 11, ISS will image the outer A ring, where it will be looking at propellers previously imaged by Cassini. Propellers are small voids in Saturn's rings created by the gravitational interaction between large ring particles and the surrounding ring. Later that day, ISS will acquire a nine-frame, wide-angle-camera mosaic of the Saturn and its ring system using the red, green, and blue filters. This mosaic will be taken as part of the Scientist For A Day program.
Cassini encounters Titan on October 14 at 04:56 UTC for the 96th time. This is the seventh and penultimate Titan flyby planned for 2013, with the last encounter scheduled for December 1. T95 has a close-approach altitude of 961 kilometers (597 miles). This flyby will allow for imaging of the north polar region inbound to the encounter and the southern anti-Saturn hemisphere of Titan on the outbound leg. Before the encounter on October 12, ISS will acquire three cloud monitoring observations. Early on October 13, ISS will ride-along with the Composite Infrared Spectrometer (CIRS) as it stares at Titan in order to acquire mid-infrared spectral and temperature information about Titan's atmosphere. During this CIRS observation, on October 13 at 13:40 UTC, Cassini will reach periapse for Rev 198 at an altitude of 1.14 million kilometers (0.71 million miles) from Saturn's cloud tops. Cassini will be 294,150 kilometers (182,780 miles) from Titan at the time. Afterward, ISS will acquire a six-frame mosaic of the north polar and northern leading hemisphere regions of Titan. This mosaic will provide more information of the distribution of lakes in the north polar region, fill-in the largest gap in ISS's map of Titan, and provide details on the shapes of some of the lakes not yet covered by RADAR SAR data. An additional frame will be taken centered on the north pole. Afterward, ISS will ride along with a CIRS far-infrared nadir-staring observation. This will be followed by a CIRS limb mapping observation. Both observations are designed to measure the distribution of temperature and minor gas abundances within Titan's atmosphere.
The Ion and Neutral Mass Spectrometer (INMS) will be the prime instrument at closest approach with RADAR riding along. RADAR will control pointing for most of the 12 hours surrounding closest approach. RADAR will acquire radiometry and scatterometry scans of Titan, which provide a variety of information about the properties of its surface including roughness and temperature. Inbound, RADAR will acquire a high-altitude SAR imaging swath across the northern trailing hemisphere, covering an area southwest of Ligeia Mare. At closest approach, INMS will directly measure the composition of Titan's upper atmosphere as Cassini makes its pass over Titan's equator. This encounter will provide information on the interaction between the solar wind and Titan's atmosphere as it will likely be outside Saturn's magnetic field at the time. A RADAR SAR imaging swath will be taken at the same time, running from Titan's north polar region south to eastern Adiri. After closest approach, RADAR will acquire a high-altitude RADAR SAR swath across the southern anti-Saturn hemisphere to almost the southern pole. This low-resolution swath will include Ontario Lacus.
After RADAR is finished with its observations, CIRS will acquire several observations of the southern hemisphere of Titan. These are designed to study changes in its atmosphere as the region approaches winter. On October 15 and 16, ISS will acquire six cloud monitoring observations of the region.
On October 16, ISS will ride along with a VIMS observation of an occultation of the star R Lyrae by Saturn's F ring. On October 18, ISS will acquire a movie of the F ring, observing its various channels and streamers created by the interaction between the ring material and the nearby moon, Prometheus. On October 19 and 21, ISS will take a movie of Saturn's B ring, looking for dust spokes. A similar ring movie will also be taken of the D ring on October 21. On October 20, ISS will look at a crescent Titan from a distance of 1.71 million kilometers (1.06 million miles). Finally on November 3, ISS will again image a crescent Titan, this time from a distance of 3.89 million kilometers (2.42 million miles).
On November 7, Cassini will reach apoapse, bringing Rev 198 to a close and starting up the next orbit, Rev 199, which will include another targeted flyby of Titan. Between November 4 and 8, Cassini will be in solar conjunction. During this time, the Sun will be nearly between Saturn and Earth and communications will be limited.
Image products created in Celestia. All dates in Coordinated Universal Time (UTC).